If the value is true, which is the default, everything received from the socket will be sent as messages to the receiving process. If the value is false (passive mode), the process must explicitly receive incoming data by calling gen_tcp:recv/2,3 or gen_udp:recv/2,3 (depending on the type of socket).

If the value is once ({active, once}), one data message from the socket will be sent to the process. To receive one more message, setopts/2 must be called again with the {active, once} option.

When using {active, once}, the socket changes behaviour automatically when data is received. This can sometimes be confusing in combination with connection oriented sockets (i.e. gen_tcp) as a socket with {active, false} behaviour reports closing differently than a socket with {active, true} behaviour. To make programming easier, a socket where the peer closed and this was detected while in {active, false} mode, will still generate the message {tcp_closed,Socket} when set to {active, once} or {active, true} mode. It is therefore safe to assume that the message {tcp_closed,Socket}, possibly followed by socket port termination (depending on the exit_on_close option) will eventually appear when a socket changes back and forth between {active, true} and {active, false} mode. However, when peer closing is detected is all up to the underlying TCP/IP stack and protocol.

Note that {active,true} mode provides no flow control; a fast sender could easily overflow the receiver with incoming messages. Use active mode only if your high-level protocol provides its own flow control (for instance, acknowledging received messages) or the amount of data exchanged is small. {active,false} mode or use of the {active, once} mode provides flow control; the other side will not be able send faster than the receiver can read.

Enable/disable permission to send broadcasts.

Determines the size of the user-level software buffer used by the driver. Not to be confused with sndbuf and recbuf options which correspond to the kernel socket buffers. It is recommended to have val(buffer) >= max(val(sndbuf),val(recbuf)). In fact, the val(buffer) is automatically set to the above maximum when sndbuf or recbuf values are set.

Normally, when an Erlang process sends to a socket, the driver will try to immediately send the data. If that fails, the driver will use any means available to queue up the message to be sent whenever the operating system says it can handle it. Setting {delay_send, true} will make all messages queue up. This makes the messages actually sent onto the network be larger but fewer. The option actually affects the scheduling of send requests versus Erlang processes instead of changing any real property of the socket. Needless to say it is an implementation specific option. Default is false.

When {active, true} delivers data on the forms port : {S, {data, [H1,..Hsz | Data]}} or term : {tcp, S, [H1..Hsz | Data]}.

Enable/disable routing bypass for outgoing messages.

By default this option is set to true.

The only reason to set it to false is if you want to continue sending data to the socket after a close has been detected, for instance if the peer has used gen_tcp:shutdown/2 to shutdown the write side.

This option is only meaningful if the binary option was specified when the socket was created. If the header option is specified, the first Size number bytes of data received from the socket will be elements of a list, and the rest of the data will be a binary given as the tail of the same list. If for example Size == 2, the data received will match [Byte1,Byte2|Binary].

Sender is forced busy if sent and enqueued data reaches the highwater mark.
Default: 8192 kB.

Restricts the socket to only use IPv6, prohibiting any IPv4 connections. This is only applicable for IPv6 sockets (option inet6).

On most platforms this option has to be set on the socket before associating it to an address. Therefore it is only reasonable to give it when creating the socket and not to use it when calling the function (setopts/2) containing this description.

The behaviour of a socket with this socket option set to true is becoming the only portable one. The original idea when IPv6 was new of using IPv6 for all traffic is now not recommended by FreeBSD (you can use {ipv6_v6only,false} to override the recommended system default value), forbidden by OpenBSD (the supported GENERIC kernel) and impossible on Windows (that has separate IPv4 and IPv6 protocol stacks). Most Linux distros still have a system default value of false. This policy shift among operating systems towards separating IPv6 from IPv4 traffic has evolved since it gradually proved hard and complicated to get a dual stack implementation correct and secure.

On some platforms the only allowed value for this option is true, e.g. OpenBSD and Windows. Trying to set this option to false when creating the socket will in this case fail.

Setting this option on platforms where it does not exist is ignored and getting this option with getopts/2 returns no value i.e the returned list will not contain an {ipv6_v6only,_} tuple. On Windows the option acually does not exist, but it is emulated as being a read-only option with the value true.

So it boils down to that setting this option to true when creating a socket will never fail except possibly (at the time of this writing) on a platform where you have customized the kernel to only allow false, which might be doable (but weird) on e.g. OpenBSD.

If you read back the option value using getopts/2 and get no value the option does not exist in the host OS and all bets are off regarding the behaviour of both an IPv6 and an IPv4 socket listening on the same port as well as for an IPv6 socket getting IPv4 traffic.

Enables/disables periodic transmission on a connected socket, when no other data is being exchanged. If the other end does not respond, the connection is considered broken and an error message will be sent to the controlling process. Default disabled.

Determines the timeout in seconds for flushing unsent data in the close/1 socket call. If the 1st component of the value tuple is false, the 2nd one is ignored, which means that close/1 returns immediately not waiting for data to be flushed. Otherwise, the 2nd component is the flushing time-out in seconds.

If the port has reached its high_watermark it will force busy onto senders. When the port data queue reaches the low_watermark callers are no longer forced busy.
Default: 4096 kB.

Received Packet is delivered as defined by Mode.

Received Packet is delivered as a list.

Received Packet is delivered as a binary.

If Boolean == true, the TCP_NODELAY option is turned on for the socket, which means that even small amounts of data will be sent immediately.

Defines the type of packets to use for a socket. The following values are valid:

Sets the max allowed length of the packet body. If the packet header indicates that the length of the packet is longer than the max allowed length, the packet is considered invalid. The same happens if the packet header is too big for the socket receive buffer.

For line oriented protocols (line,http*), option packet_size also guarantees that lines up to the indicated length are accepted and not considered invalid due to internal buffer limitations.

Set the protocol-defined priority for all packets to be sent on this socket.

See below.

Sets the max number of UDP packets to read without intervention from the socket when data is available. When this many packets have been read and delivered to the destination process, new packets are not read until a new notification of available data has arrived. The default is 5, and if this parameter is set too high the system can become unresponsive due to UDP packet flooding.

Gives the size of the receive buffer to use for the socket.

Allows or disallows local reuse of port numbers. By default, reuse is disallowed.

Only allowed for connection oriented sockets.

Specifies a longest time to wait for a send operation to be accepted by the underlying TCP stack. When the limit is exceeded, the send operation will return {error,timeout}. How much of a packet that actually got sent is unknown, why the socket should be closed whenever a timeout has occurred (see send_timeout_close). Default is infinity.

Only allowed for connection oriented sockets.

Used together with send_timeout to specify whether the socket will be automatically closed when the send operation returns {error,timeout}. The recommended setting is true which will automatically close the socket. Default is false due to backward compatibility.

Gives the size of the send buffer to use for the socket.

Sets the SO_PRIORITY socket level option on platforms where this is implemented. The behaviour and allowed range varies on different systems. The option is ignored on platforms where the option is not implemented. Use with caution.

Sets IP_TOS IP level options on platforms where this is implemented. The behaviour and allowed range varies on different systems. The option is ignored on platforms where the option is not implemented. Use with caution.